Vascular development requires a complex series of events during which endothelial cells differentiate from angioblast progenitors and assemble into the dorsal aorta and the cardinal vein, comprising the original circulatory loop. Arteries and veins are morphologically, functionally and molecularly different. How this distinction is established at cellular and molecular levels in the early stage of vasculogenesis is largely unknown. New genetic evidence from zebra fish studies suggests that the Notch signal transduction pathway is required for vascular specification. The gridlock gene, encoding an Enhancer of Split-related bHLH protein, mediates Notch signaling to promote development of the embryonic arteries. The research proposal explains how cellular and genetic analysis are used in zebra fish to understand the role of notch-gridlock pathway in regulating arterial/venous endothelial fate of angioblasts. Specially, this proposal first aims to identify potential cell-cell interactions by following angioblast migration. Next, experiments are designed to examine expression pattern of Notch signaling components in vascular development, as well as to determine the fate of individual angioblasts that are incapable of normal Notch signaling. The final experiments are proposed to test the mechanism by which gridlock mediates Notch signaling via a negative feedback loop. As fundamentals of vascular development are conserved between vertebrates and mammals, understanding the notch-gridlock signaling pathway in zebrafish will provide insights into mechanisms of vascular development as well as advance our understanding of etiologies of many human arterial diseases.